Spherical antenna pattern(s) from antenna(s) arranged in a two-dimensional plane for use in RFID tags and labels

ABSTRACT

The present invention introduces an RFID system that comprises a two-dimensional antenna configuration having a dipole in combination with a loop antenna or a second dipole with their dipole axes at approximately a 90° angle to one another. The pattern of two dipole antennas combined with the pattern of a single loop antenna, allows the sequential combination of the patterns to represent a nearly spherical antenna pattern in three dimensions, while the antennas reside in a two-dimensional plane. The presence of multiple antennas also allows for a method to switch among the multiple antennas to find the antenna or combination of antennas that create(s) the strongest RF communication link thereby enabling more efficient transmitted power or more directional reception coverage. Improving efficiency can be further improved by taking this concept one step further by adding multiple antennas into the interrogator unit. Now the possibility exists whereby the best RF communication possible can be established between the interrogator and transceiver units by switching to the best antenna combination that would most efficiently pass an RF signal between the interrogator and the transceiver before and after which any of the antennas are now free to be used for trickle charging the chargeable battery cell(s) that reside in the transceiver unit.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation to U.S. patent application Ser. No. 08/422,007,filed Apr. 11, 1995 now U.S. Pat. No. 5.572,226, which is a continuationto U.S. patent application Ser. No. 08/137,699, filed Nov. 14, 1993 nowabandoned, which is a continuation of U.S. patent application Ser. No.07/884,507, filed May 15th , 1992 now abandoned.

FIELD OF THE INVENTION

This invention relates generally to the field of radio frequencytransceiver systems and in particular to radio frequency identification(RFID) devices (or tags). More specifically, a nearly spherical antennapattern is developed from multiple antennas in a two-dimensional planeas well as an antenna switching method to utilize the multiple antennas.

BACKGROUND OF THE INVENTION

The field of RFID systems has evolved over the past years from systemsdeveloped in relatively large packages (size of a cigarette package)containing battery powered transmission/receiving circuitry ortransceiver, such as the identification system disclosed in U.S. Pat.No. 4,274,083, to passive systems (the size of a credit card) in whichthe transceiver receives its power from the base station orinterrogator, such as the identification system disclosed in U.S. Pat.No. 4,654,658.

Although a low-cost RFID system having a battery powered transceiverallows for greater distance between the interrogator and the transceiverunits, the physical size of the transceiver unit has pushed the industrytowards the passive transceiver technology as disclosed in U.S. Pat.Nos. 4,654,658 and 4,730,188. However, the limit in RF communicationdistance between transceiver and interrogator in low-cost passivesystems is in the 2 foot or less range if reasonable accuracy ofinformation collected is to be achieved and low power, low-costinterrogators are to be used.

U.S. Pat. No. 4,724,427 discloses a passive RFID transceiver as well asa hybrid battery operated version which addresses both types of RFIDsystems.

The memory used in RFID systems depends on the type of system usedwherein, most passive systems use a programmable read only memory (PROM)of some sort that will retain its programmed data even when power isdisrupted or removed while a battery backed system may use any memorytype, including random access memory (RAM), that requires a continuouspower source to retain programmed data.

Though an RFID system may vary in transceiver type, the memory used andcircuitry configuration, all RFID systems have a common characteristic:that being a receive/send antenna(s) component. The present inventiondiscloses the development of a three-dimensional spherical antennapattern that is constructed from elements arranged in a two-dimensionalplane, a method for switching to various antenna elements and passiverecharging of secondary battery cell(s) powering RFID transceiver tags,each of which will allow improved operation of compatible RFID systemsthat will readily be recognized by one skilled in the art in accordancewith the present invention described herein. Also, all U.S. patentscited herein are hereby incorporated by reference.

SUMMARY OF THE INVENTION

The present invention introduces an RFID system that comprises atwo-dimensional antenna configuration that can create athree-dimensional spherical antenna pattern, an antenna switching methodand a method to utilize an RF signal for trickle charging passivebattery cells resident in the tag transceiver unit.

The two-dimensional antenna configuration is composed of a combinationof one or two dipole antennas and/or a single loop antenna. If twodipoles are used it is preferred to have them arranged such that theiraxes at approximately a 90° angle to one another. If a dipole and a loopare used, any desired antenna orientation in the plane may be selected.It is well known that a simple dipole antenna driven by an RF voltagesource produces a toroidal shaped electromagnetic field centered aboutthe dipole axis. Therefore, when the pattern of a dipole antenna axis iscombined with another dipole or the toroidal pattern of a single loopantenna (which performs as if it were a dipole with its axis through thecenter of the loop), a nearly spherical antenna pattern results whilethe physical elements exist essentially in a two-dimensional plane. Whenall three elements are combined, with the second dipole's axis atapproximately a 90° angle to the first dipole's axis, the pattern isfurther filled in and the antennas remain in the two-dimensional plane.

Because the tag transceiver unit now consists of multiple antennas, italso allows for a method to switch among the multiple antennas to findthe antenna or combination of antennas that create(s) the strongest RFcommunication link thereby enabling more efficient transmitted power ormore directional reception coverage. Improving efficiency can be furtherimproved by taking this concept one step further by adding multipleantennas into the interrogator unit. Now the possibility exists wherebythe best RF communication possible can be established between theinterrogator and transceiver units by switching to the best antennacombination that would most efficiently pass an RF signal between theinterrogator and the transceiver. The antenna switching concept wouldwork by having multiple antennas in either the interrogator unit or thetag transceiver unit or both.

The presence of multiple antennas creates an additional advantage inthat when the antennas are not being used for communication, they arenow free to be used for trickle charging the chargeable battery cell(s)that reside in the transceiver unit.

Also, the presence of multiple antennas creates an additional advantagein that each antenna combination has a different efficiency due todifferent orientations of polarizations, and is also helpful inminimizing the effects of multipath interference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a planar top view of an RFID transceiver unit containingtwo dipole antennas in combination with a loop antenna and a signalsource or receiver;

FIGS. 2a-2c depict examples of some of the possible antennaconfigurations that, when positioned in place of the dipoles andoriented with a loop antenna as shown in FIG. 1, will provide sphericalantenna pattern coverage; and

FIGS. 3a and 3b depict multiple antenna variations for use in a RFIDsystem so that when contained in either the interrogator unit and fortag transceiver unit, antenna switching is possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several concepts are intertwined in the drawings depicted in FIGS. 1through FIGS. 3a and 3b as will be described hereinafter.

Referring now to FIG. 1, dipole antennas 11 and 12 have been processedonto substrate 10 which contains an RFID tag transceiver unit. Dipoleantennas 11 and 12 have been processed onto substrate 10 atapproximately 90° angles to one another. Loop antenna 13 has also beenprocessed onto substrate 10 at approximately a 90° to the axis ofantenna 12. RF source 14 has also been processed onto substrate 10 torepresent a signal source or a receiver input depending on the mode ofoperation the transceiver unit is in.

Since an RF voltage will produce a toroidal shaped electromagnetic fieldcentered about a dipole antenna, the particular antenna configurationdepicted in FIG. 1 will effectively achieve a nearly sphericalelectromagnetic field pattern when combined by switching them one at atime to an RF transmitter or receiver. This combination of the threeantenna patterns reduces or eliminates the lack of field on-axis of thedipole antennas, while using only antenna geometries lying within atwo-dimensional plane of substrate 10. Since the combined coverage isapproximately spherical, the orientation between the RFID transceiver(or tag) and the interrogator(s) becomes substantially irrelevant. Andalthough the loop is shown in a rectangular layout, it could as welltake other forms, such as circular.

The dipole antennas of FIG. 1 may be replaced by an unlimited number ofconfigurations, such as the antenna configurations presented in FIGS.2a-2c, with the main stipulation being that to gain the optimumspherical electromagnetic field pattern it is preferred to have any twodipole antennas placed at approximately 90° to one another (however,they need not be if so desired). The many antenna configurations thatone skilled in the art could come up with and that are arranged in theorientation as depicted in FIG. 1 may provide for a more optimumspherical pattern coverage or for achieving different impedancecharacteristics. Although the orientation of FIG. 1 is preferred, theintent of the present invention is to construe the concept of obtainingspherical pattern coverage from antennas lying in a two-dimensionalplane.

Now that an antenna configuration that lies in a two-dimensional planecan be achieved with two dipole antennas and a loop antenna,communication within range between an interrogator and a planar RFID tagtransceiver can be established no matter what the physical orientationbetween the two. The presence of multiple antennas allows for a secondconcept which will be referred to as antenna switching whereby, a higheroperating efficiency is obtained by switching mechanically orelectrically to the best combination of antennas (or to the singleantenna) which presents the strongest RF communication link betweeninterrogator and transceiver units.

For purposes of the antenna switching concept, the transceiver unit mustcontain circuitry capable of interpreting a command to switch and thento switch among the antennas. Digital logic could be used in conjunctionwith memory means, such as Static Random Access Memory (SRAM), DynamicRandom Access (DRAM) or the like. The presence of logic and volatilememory will also require a power supply means, such as rechargeablebattery cells that are either permanently fabricated onto substrate 10or separate cells, depending on one skilled in the art's choice ofdesign or as technology progresses.

An embodiment of antenna switching is discussed in light of FIGS. 1 and3a. Referring now to FIG. 3a, an RFID interrogator unit's antenna systemnow contains multiple antennas 31, 32 and 33. The interrogator computercircuitry, under software control, switches to its first antenna, forexample antenna 31, for transmitting a wake up call to the RFIDtransceiver's first antenna, say antenna 11 in FIG. 1. The transceivercircuitry wakes up and switches among its antennas 11, 12 and 13,thereby memorizing the power from each antenna by storing thisinformation in the transceiver's resident memory. The interrogator thenswitches to its second antenna, say antenna 32, and again thetransceiver switches among its antennas 11, 12 and 13, and stores therelative signal strength of each. This process continues until allpossible antenna combinations between the interrogator and thetransceiver are tried and memorized. Then logic circuitryresiding ineither the tag transceiver or interrogator units selects the bestinterrogator/transceiver RF link and switches to that combination forensuing communications.

Antenna switching could also be accomplished by having multiple antennasresident in either the interrogator or the transceiver and follow theprocedure just described, thereby establishing all possible antennacombinations. It is also conceivable to have other multiple antennaconfigurations resident in the interrogator unit like the configurationshown in FIG. 3b. In FIG. 3b, dipole antennas 34 and 35 are oriented inmanner corresponding to transceiver antennas 11 and 12 of FIG. 1.Likewise, loop antenna 36 is oriented in a manner corresponding totransceiver loop antenna 13 of FIG. 1.

Another advantage of having multiple antennas that allows antennaswitching in the transceiver unit is that they are available for thepurposes of trickle charging the rechargeable battery resident in thetransceiver unit.

It is to be understood that although the present invention has beendescribed in several embodiments, various modifications known to thoseskilled in the art, such as having any number of antennas present in theinterrogator or transceiver units, may be made without departing fromthe invention as recited in the several claims appended hereto.

I claim:
 1. A radio frequency (RF) transceiver system comprising firstand second sending/receiving units wherein at least one of the unitscontains multiple antennas constructed in a planar arrangement, saidplanar arrangement creating a spherical electromagnetic antenna pattern,wherein said multiple antennas comprise at least one dipole antenna andconsist of a single loop antenna formed in a planar arrangement therebycreating a toroidal shaped electromagnetic field centered about eachantenna axis and resulting in said spherical electromagnetic pattern. 2.The RF transceiver system of claim 1 wherein both said first and secondunits comprise said multiple antennas constructed in a planararrangement.
 3. The RF transceiver system of claim 1 wherein said firstunit comprises said multiple antennas constructed in a planararrangement.
 4. The RF transceiver system of claim 1 wherein said secondunit comprises said multiple antennas constructed in a planararrangement.
 5. The RF transceiver system of claim 1 wherein said firstunit is an interrogator unit comprising said multiple antennas.
 6. TheRF transceiver system of claim 1 wherein said RF transceiver systemcomprises a radio frequency identification system.
 7. The RF transceiversystem of claim 1 wherein said second unit is a transceiver unit havingsaid multiple antennas.
 8. The RF transceiver system of claim 7 whereinsaid transceiver unit comprises decision making circuitry, volatilememory and a power source.